Output of the sleep circuit. a, pdfr-GAL4 marks output cells of the LNv circuit. Left panels show reconstructed pdfr-GAL4;UAS-mCD8-GFP adult brain stained with anti-PDF (1:1000) and visualized with confocal microscopy. PDF is shown in magenta, GFP in green, overlap in white. Scale bar = 150 μm. Right panels show a 37 μ section of a pdfr-GAL4;UAS-mCD8-GFP adult brain stained with anti-Per (1:100) and visualized with confocal microscopy. Per is shown in magenta, GFP in green, overlap in white. Arrows indicate clock cells. Scale bar = 20 μm. b, Model of the Drosophila sleep circuit. Light, and perhaps other arousal cues, activate l-LNvs which release PDF onto s-LNvs that project to other clock cells and also send dorsal projections that pass by pdfr-GAL4 positive cells groups such as the ellipsoid bodies that are involved in control of activity. Both l- and s-LNvs express GABAA receptors, allowing sleep promoting GABAergic neurons to suppress wakefulness.